This invention relates to hardness testing devices which utilize the rebound of a falling object, sometimes referred to as a striker, from the surface of a test specimen to measure the hardness of said specimen. More particularily, this invention relates to a type of harness tester which utilizes a free falling striker contained in a vertical tubular structure with striker releasing means at the top, and striker retaining means at the bottom.
Previous hardness testers of this type have suffered from several disadvantages;
To achieve accurate hardness readouts in terms of striker rebound it is desirable that the striker be unencumbered by friction. In previous rebound type hardness testers the striker was usually operating within the confines of a closely fitting tubular structure which often resulted in inconsistent striker rebounds due to a high probability of random frictional contact between the striker and said tubular structure, this condition being aggrevated by deviations of the tubular structure from the perfectly vertical position.
To operate a rebound type hardness tester repeatedly, the striker must be returned to its release position above the test specimen. To achieve consistent rebounds of the striker, it is most desirable to release said striker with substantially zero velocity at a specific height above the test specimen. Some prior art instruments returned and released the striker by means of complex lifting and releasing mechanisms. Others, who relied on the simple expedient of returning the striker to its release position by inverting the instrument, utilized the inertia of the striker to engage it in its release position by means of spring loaded latches, levers, or cams, all of which can adversely affect the velocity of the striker at the time of release, either by accelerating the striker due to operator dexterity in overcoming spring pressure or friction of the release mechanism, or by retarding striker velocity due to friction of the striker against a fixed or movable surface of the instrument during release.
To test the hardness of small test specimens it is desirable that the mass of the striker be kept to a minimum consistent only with adequate rebound indication. Previous hardness testers employed strikers of massive proportions, their shape, size and mass being dictated by the means utilized to release and to retain said strikers.